实现跟随手指移动的小球

private Paint mPaint;

private int cx = 200;
private int cy = 200;

public CustomCircleView(Context context) {
    super(context);
}
public CustomCircleView(Context context, AttributeSet attrs) {
    super(context, attrs);
    mPaint = new Paint();
    mPaint.setColor(Color.RED);
    //mPaint.setStyle(Paint.Style.STROKE);
    mPaint.setAntiAlias(true);
    mPaint.setStrokeWidth(10);
}
public CustomCircleView(Context context, AttributeSet attrs, int defStyleAttr) {
    super(context, attrs, defStyleAttr);
}
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    super.onMeasure(widthMeasureSpec, heightMeasureSpec);
}
@Override
public boolean onTouchEvent(MotionEvent event) {
    switch (event.getAction()){
        case MotionEvent.ACTION_DOWN:
            System.out.println("event ACTION_DOWN = " + event.getRawX() + "  " + event.getRawY());
            break;
        case MotionEvent.ACTION_MOVE:
            System.out.println("event ACTION_MOVE = " + event.getRawX() + "  " + event.getRawY());
            cx = (int) event.getX() ;
            cy =(int) event.getY();
            //刷新
            invalidate();
            break;
        case MotionEvent.ACTION_UP:
            System.out.println("event ACTION_UP = " + event.getRawX() + "  " + event.getRawY());
            break;
    }
    return true;
}
@Override
protected void onDraw(Canvas canvas) {
    super.onDraw(canvas);
    canvas.drawCircle(cx,cy,200,mPaint);
}